1. Field of the Invention
In general, the present invention relates to a new generation of hydrogen permeable membranes that can be used to separate pure hydrogen from mixed gas sources. More particularly, the present invention relates to the physical structure of such hydrogen permeable membranes.
2. Prior Art Description
In the industry, there are many applications for the use of molecular hydrogen. However, in many common processes that produce hydrogen, the hydrogen gas produced is not pure. Rather, when hydrogen is produced, the resultant gas is often contaminated with water vapor, carbon monoxide, hydrocarbons and/or other contaminants. In many instances, such as fueling a proton exchange fuel cell, it is necessary to have ultra pure hydrogen. In the art, ultra pure hydrogen is commonly considered to be hydrogen having purity levels of at least 99.999%. In order to achieve such purity levels, hydrogen gas must be actively separated from its contaminants.
In the prior art, one of the most common ways to purify contaminated hydrogen gas is to pass the gas through a pressure swing absorption system that effectively absorbs most of the contaminating gases and lets the hydrogen gas pass through with only a small pressure drop. This is an energy inefficient technology that operates best in large plant operations.
When hydrogen gas is purified to an ultra pure state, it is passed through a membrane of hydrogen permeable material, such as palladium or a palladium alloy. In order for hydrogen to permeate through a palladium-based membrane at a practical rate, there must be enough thermal energy present to disassociate molecular hydrogen in the presence of palladium into atomic hydrogen on the surface of the membrane. The palladium-based membrane then absorbs the atomic hydrogen into its interior volume. The atomic hydrogen permeates through the membrane from a high pressure side of the membrane to a low pressure side of the membrane. Once at the low pressure side of the membrane, the atomic hydrogen recombines to form molecular hydrogen which can either leave the surface of the membrane or again disassociate into two hydrogen atoms, either of which or both may be reabsorbed into the bulk of the membrane material. Once in the bulk of the membrane material, the hydrogen atom can emerge from the bulk on either side of the membrane. The direction of the net hydrogen (H2) gas flow after separation and recombination is determined by which side of the membrane has more hydrogen atoms dropping into the bulk. For a palladium membrane whose surface is the same on both sides, the number of hydrogen atoms that split and drop into the bulk is predominately determined by the pressure of the hydrogen gas. Consequently, in order to maintain a flow of purified hydrogen, a pressure differential must be maintained between the two sides of the membrane. The purified side of the membrane is kept at a lower pressure than the contaminated side of the membrane. This ensures that hydrogen gas has a bias that moves it through the membrane. Relying upon a pressure differential to move hydrogen through a membrane has many limitations that detract from both the efficiency and running life of prior art hydrogen separators.
One way to improve a hydrogen permeable membrane would be to make the contaminated gas side of the membrane more reactive to splitting the hydrogen gas molecule than the pure hydrogen gas side. In this manner, the combination of the two membrane surfaces would act as a one-way valve, thereby reducing the need for a differential hydrogen pressure to prevent backward hydrogen flow. The net flow of hydrogen gas depends only on the difference in the catalytic ability of the opposite sides of the membrane to split the hydrogen molecule.
Attempts have been made to produce membranes having opposing sides that embody different hydrogen molecule disassociation characteristics. Many of these attempts include producing membranes from layers of different hydrogen permeable materials. Unsuccessful attempts included forming membranes where the contaminated gas side of the membrane is made from a palladium alloy, and the opposite side of the membrane is made from niobium or tantalum. In theory, such multilayer membranes were expected to work. Palladium is a catalyst that promotes the splitting of hydrogen molecules (H2) into two atoms of hydrogen (H+H). Metals, such as niobium, tantalum and vanadium are permeable to atomic hydrogen but are not effective catalysts for disassociating molecular hydrogen into atomic hydrogen. Thus, hydrogen gas would dissociate into atomic hydrogen as it contacted the palladium alloy on the contaminated side of the membrane. The atomic hydrogen would pass through the membrane and would immediately recombine into molecular hydrogen once out of the membrane on the opposite side. In a reverse flow situation, molecular hydrogen would contact the niobium or tantalum layer. Since these metals do not promote the disassociation of molecular hydrogen into atomic hydrogen, the molecular hydrogen remains as molecular hydrogen. The molecular hydrogen therefore would not be able to pass into the structure of the membrane. Consequently, hydrogen flows far more efficiently from the palladium layer to the niobium/tantalum layer than in reverse.
Although such membranes were good in theory, they did not work in reality. Hydrogen separation membranes operate at high temperatures. Once a palladium membrane was coated with niobium or tantalum, the atoms of niobium/tantalum migrate into the palladium. At the high operating temperatures of a hydrogen separator, the niobium/tantalum atoms diffuse into the palladium atoms, eventually creating a homogenous alloy. Once the niobium and tantalum atoms are dispersed in the palladium, the benefits of a separate niobium/tantalum layer are lost.
The problem of metal atom diffusion has been addressed by coating opposite sides of a neutral porous substrate with palladium and niobium or tantalum. However, the porous substrate significantly detracts from the ability of hydrogen to efficiently pass through the membrane. Furthermore, due to differences in thermal expansion between the hydrogen permeable metals and the substrate, membranes with substrates tend to have shortened operational lives.
A need therefore exists for a way to create a hydrogen permeable membrane using different hydrogen permeable metals without a substrate and without having the hydrogen permeable metals interdiffuse. This need is met by the present invention as described and claimed below.